1. Field of the Invention
The present invention relates to a current feedback operational amplifier, and particularly to a current feedback operational amplifier in which a negative feedback loop is formed by input ends and an input pair of current switches for enhancing gains.
2. Description of Related Art
In general, the slew rate and bandwidth of a current feedback operational amplifier are larger than that of a voltage feedback operational amplifier, and is more suitable for a low power operation. Therefore, the current feedback operational amplifier is usually used in a line driver to drive a large load.
FIG. 1 shows a prior art circuit of a current feedback operational amplifier, comprises a CCII+ (second generation current conveyor) 11 and an output stage 12. The current conveyor 11 has two types: class A and class A/B. The input of the class A current conveyor 11 has a more accurate frequency response and more suitable for a low voltage operation. The class A/B current conveyor 15 has a better slew rate and S/N ratio (signal to noise ratio).
FIG. 2(a) shows a prior art circuit of a class A/B circuit conveyor, disclosed by Thomas Kaulberg in xe2x80x9cA CMOS Current-Mode Operational Amplifier,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol.28, No.7, pp.849-852, July 1993. Besides, U.S. Pat. No. 6,028,479, titled xe2x80x9cLOW VOLTAGE TRANSMISSION LINE DRIVERxe2x80x9d also discloses a similar fully differential circuit.
An input signal of an ideal current conveyor is current; therefore, the input impedance at a terminal X is zero. In the circuit shown in FIG. 2, the input impedance at terminal X is (gm2+gm4)xe2x88x921. For achieving a lower input impedance, the aspect ratio and bias current of transistors M2 and M4 are designed as larger ones, and therefore occupy a large area. Besides, the problems of a large offset voltage and gain error still exist. For improving the performance of the prior art circuit, a feedback loop is established to reduce the input impedance, shown in FIG. 2(b). The prior art circuit is disclosed by Eyad Abou-Allam in xe2x80x9cA 200 MHz Steered Current Operational Amplifier in 1.2 xcexcm CMOS Technology,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol.32, No.2, February 1997. According to a feedback theorem, the input impedance of the prior art circuit is reduced as (1+Av)xe2x88x921 times as the original one, and that is (gm1)xe2x88x921/(1+Av). However, the prior art circuit belongs to a class A current conveyor, and cannot be applied in a class A/B current conveyor.
A first object of the present invention is to reduce the design complexity of a current feedback operational amplifier.
A second object of the present invention is to reduce input impedances of a current feedback operational amplifier.
A third object of the present invention is to improve aspect ratio and bias current of transistors included in a current feedback operational amplifier.
A fourth object of the present invention is to improve the problems of offset voltage and gain error happened occurring in a current feedback operational amplifier.
For achieving the above objects, the present invention proposes a current feedback operational amplifier, in which a first and second input ends are connected to a first amplifier which transmits an output to the gate terminals of at least one input pair of current switches, and the source terminal of one of the input pair of current switches is connected to the first input end. Therefore, a negative feedback loop is established by the first amplifier and input pair of current switches. By means of the negative feedback loop, the input impedance, offset voltage and gain error are all reduced. The input impedance of the present invention is (gm2+gm4)xe2x88x921/(1+A), wherein A is the gain of the amplifier, gm2 and gm4 are transconductances of transistors M2 and M4, respectively. The input impedance is reduced as 1/1+A times as the original one. Therefore, the aspect ratio of transistors M2 and M4 is reduced.
The first embodiment of the current feedback operational amplifier has a first and a second input end, comprising a current conveyor and an output stage. The current conveyor includes a first amplifier and at least one input pair of current switches, characterized in that said first and second input ends function as inputs of said first amplifier. The output of said first amplifier is electrically connected to gate terminals of said at least one input pair of current switches, and a source terminal of one transistor of said at least one input pair of current switches is connected to said first input end. The output stage is connected to said current conveyor for enhancing the output driving capability. Therefore, a negative feedback loop is established to enhance gains and to reduce input impedance.
The second embodiment of the current feedback operational amplifier has a first and a second input end and a first output end, comprising current conveyor and an output stage. The current conveyor includes a first amplifier, input pair of current switches and a current mirror. The input end of the first amplifier is connected to said first and second input ends. The gate terminals of input pair of current switches are connected to the output of said first amplifier, and a source terminal of one transistor of said input pair of current switches is connected to said first input end. The source terminals of the current mirror are connected to source terminals of said input pair of current switches. The output stage is connected to said current conveyor for enhancing output driving capability and transmitting an output to said first output end.
The third embodiment of the current feedback operational amplifier has a first and a second input end and a first and a second output end, comprising a current conveyor and an output stage. The current conveyor includes a first amplifier, two input pairs of current switches and two current mirrors. The input end of the first amplifier is connected to said first and second input ends. The gate terminals of the two input pairs of current switches are connected to the output of said first amplifier respectively, and source terminals of one transistor of each said two input pairs of current switches connected to said first and second input ends respectively. The source terminals of-the two current mirrors are connected to the source terminals of said two input pair of current switches. The output stage is connected to said current conveyor for enhancing output driving capability and transmitting an output to said first and second output ends.
Besides, some transistors of the output stage are designed by a negative feedback loop for enhancing gain.